<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .app{
      width: 300px;
      height: 300px;
      background-color: royalblue;
      display: flex;
      align-items: center;
    }
    .box{
      width: 50px;
      height: 50px;
      background-color: wheat;
    }
  </style>
</head>
<body>
  <div class="app">
    <div class="box"></div>
  </div>

  <script>
    const a = {}  // 字面量
    const b = new Object()
    const c = JSON.parse("{}")

    // vDom  virtualDom
    document.querySelector()
    const dom = {

    }
  </script>
</body>
</html>
